Ohmic contacts on rare earth chalcogenides

ABSTRACT

GOOD OHMIC CONTACTS ARE MADE TO RARE EARTH CHALCOGENIDE CRYSTALS. A LOW MELTING POINT ALLOY IS PREPARED FROM RARE EARTH ELEMENTS AND AT LEAST ONE CONDUCTIVE METAL. THE ALLOY IS MELTED ON THE SURFACE OF THE CRYSTAL AND CONDUCTIVE LEADS ATTACHED THERETO BY SOLDERING WITH AN INDIUM SOLDER.

April 13, 1971 GAMBINQ ET AL. 3,574,676

OHMIC CONTACTS ON RARE EARTH CHALCO GENIDES Filed Sept. 19, 1968 DIFFUS|0N LAYER Hmmcnc v EU CHALCOGENIDE FIG.1I

F I G. 2

INVENTORS RICHARD J. GAMBINO STEPHEN VON MOLNAR ATTORNEY United States Patent O 3,574,676 OHMIC CONTACTS N RARE EARTH CHALCOGENIDES Richard J. Gambino, Yorktown Heights, and Stephen von Molnar, Ossining, N.Y., assignors to International Business Machines Corporation, Armonk, N.Y.

Filed Sept. 19, 1968, Ser. No. 760,900 Int. Cl. C23f 17/00; 'C23c 13/02; C07f 11/00 U.S. Cl. 117-201 13 Claims ABSTRACT OF THE DISCLOSURE Good ohmic contacts are made to rare earth chalcogenide crystals. A low melting point alloy is prepared from rare earth elements and at least one conductive metal. The alloy is melted on the surface of the crystal and conductive leads attached thereto by soldering with an indium solder.

FIELD OF THE INVENTION The invention relates to a method of aflixing ohmic contacts to ferromagnetic rare earth chalcogenide crystals.

BACKGROUND OF THE INVENTION Rare earth chalcogenide compositions are becoming increasingly important in the fields of ferromagnetic semiconductors, thermoelectric devices, magneto-optical devices, magneto-resistive devices and the like. The importance of these materials have. increased with the discovery that the Curie temperature thereof can now be controlled and may be expanded far above the liquid nitrogen temperature, making their use much more convenient than in the past. A more detailed listing of rare earth chalcogenides and the devices in which they find utility is set forth in the following commonly assigned patents and patent applications; 3,353,907; 3,371,042; 3,370,342; 3,370,924; and Ser. No. 411,525 and now Pat, No. 3,418,036.

A limiting factor for not exploiting the potential uses of these materials in new devices, e.g., ferromagnetic semiconductor devices, transducers and the like has been the inability of producing good ohmic contacts thereon. The concept of making ohmic contacts on semiconductor materials by soldering with suitable alloys is well known. However, the prior art method and compositions for applying such contacts on the materials contemplated herein have not been met with any significant degree of success. This is understandable since there are no rules or principles for selecting alloys suitable for particular materials, one must therefore depend on empirical findings. Pat. No. 3,370,342, issued Feb. 27, 1968 and assigned to a common assignee, discloses one such empirical formulation, viz, a 60:40 Bi-Cd eutectic, for applying ohmic contacts to rare earth chalcogenides with varying success.

SUMMARY OF THE INVENTION Good ohmic contacts are affixed to the surfaces of rare earth chalcogenide ferromagnetic semiconductor compositions by depositing thereon a low melting alloy comprising a trivalent rare earth element and a conductive metal element. The low melting alloy is melted on the surface of a rare earth chalcogenide composition having the general formula Eu RE A where RE is a trivalent rare earth element, A is a chalcogen, and x31. The melted alloy is heated to partially diffuse the same into the body of the rare earth chalcogenide. Copper lead wires can then be soldered to the alloy dot with an indium solder.

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OBJECTS OF THE INVENTION An object of the invention is to provide low melting point rare earth metal alloys for making good ohmic contacts to rare earth chalcogenide crystals.

Another object of the invention is to provide a method for making good ohmic contacts to rare earth chalcogenide crystals with low melting rare earth metal alloys.

The foregoing and other objects, features and advantages of the invention will be apparent from the drawings and the following more particular description of preferred embodiments of the invention.

FIG. 1 depicts the semiconductive rare earth chalcogenide body with an ohmic contact thereon.

FIG. 2 is a graph showing the I-V charatceristics of ohmic contacts made by this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Accoring to this invention good ohmic contacts are made on rare earth chalcogenides such as EuO, EuS, EuSe, and to rare earth chalcogenides having the general formula Eu RE A such as disclosed in Pat. No. 3,371,041 to F. Holtzberg et al. To effect such ohmic contacts, alloys of a trivalent rare earth element and a metal having good electrical conducting properties are used.

The alloys are obtained by mixing weighted amounts of the trivalent rare earth elements such as Ce, Pr, Nd, Gd, Tb, Dy, Hu, Y, Er, Tm, and Lu, and a metal selected from at least one of the following; Ag, Au, Cu, Zn, Ni, Co, Pd and Pt. These mixtures are melted by conventional arc melting techniques with a tungsten electrode and a water-cooled copper hearth in an inert atmosphere such as argon.

The alloys may also be obtained by melting a mixture of the constituent elements in a suitable container, such as tantalum, graphite, or A1 0 and by heating to a temperature sufiicient to melt the higher melting point element. The components of the alloy may be in powdered or granular form or may be prepared in any manner that would provide for its insertion into the container prior to heating the latter to fuse the two materials. The eutectic composition is heated inductively in an inert atmosphere, e.g., argon,

The invention is further illustrated by the following preferred embodiments. While the following examples of the invention relate to a few representative rare earth elements and alloys of suitable conductive metal, it should be made clear that it is the intent of the inventors that alloys of other trivalent rare earth elements and other suitable conductive metals may be included as part of their invention.

EXAMPLE 1 A powdered mixture consisting of 76% by weight of La and 24% by weight of Ag is placed in a tantalum crucible. The crucible and its charge is placed in an inductively heated furnace which is purged with argon, and is heated at a temperature of about 1000 C. for about 15 minutes. The resulting eutectic composition has a distinct melting point at 518 C.

Ohmic contacts are effected in the following manner. Small amounts, e.g., about .01 gram, of the alloy prepared above is deposited on the cleavage faces of a single crystal of the rare earth chalcogenides, e.g., B11039 Gd M1 Se crystal, which is placed on a conventional tantalum strip heater. The heater is capable of attaining temperatures sufficient to melt the alloy. Heating of the alloy and crystal is performed in an enclosed system purged of oxygen by an inert gas, e.g., argon, to prevent oxidation of the chalcogenide and alloy. The alloy is heated to its melting point temperature and heating at this temperature is continued until the alloy partially diffuses into the chalcog-enide crystal as seen in FIG. 1. The time of heating is about 15 minutes. The alloy melt is seen to immediately wet the surface of the crystal. Conductive lead wires, e.g., copper wires, are then soldered to the alloy dots with an indium solder.

The current-voltage characteristics of the contacts were measured at several temperatures between 4.2 K. and room temperature. FIG. 2 shows the current to be linearly dependent on the voltage at all temperatures measured, a clear indication that the contacts are ohmic.

EXAMPLE 2 A mixture consisting of 79.5% by weight of La in the form of rods and 20.5% of Au by weight in the form of foil is placed on the hearth plate of a conventional DC. are melting apparatus. The melting chamber of the arc melting apparatus is evacuated and filled with argon gas. An arc is struck between the hearth plate and the tungsten electrode of the apparatus and the mixture is melted for about 1 minute by positioning the tungsten electrode above it. The resulting button-shaped ingot is turned over, top for bottom and remelted by the same procedure. The ingot is remelted at least three times in this manner, being inverted between each melting stop, to insure homogeneity. The alloy has a melting point of 561 C. Ohmic contacts are aifixed to the semiconductor as indicated in Example 1. A similar I-V characteristic curve is obtained as that shown in FIG. 2.

EXAMPLE 3 The method of Example 1 is repeated except that a mixture of 86% by weight of La and 14% by weight of Cu is used. A melting point of 468 C. is observed for the resulting product. Ohmic contacts are afiixed as indicated in Example 1. Similarly, I-V characteristics such as shown in FIG. 2 are obtained.

Other illustrative embodiments of the invention are prepared according to the methods illustrated in the above examples and are listed in the ensuing table. The table gives the percentages by weight of the components together with the melting points of the resulting alloys.

Rare earth Weight, Conductive Weight, M.P. temperature element percent metal percent of alloy, C.

88. 5 Zn 11. 527 84. N1 16. 0 485 84. 0 C0 16. 0 440 82. Cu 17. 5 472 77. 9 All 22. 1 600 93. 0 Ni 7. 0 480 78. 0 Ag 22.0 582 74. 0 Cu 26.0 760 In summary there has been described a method of aflixing good ohmic contacts to ferromagnetic rare earth chalcogenide semiconductor bodies. A number of low melting rare earth element-conductive metal alloys are prepared which are found to make excellent ohmic contacts to ferromagnetic semiconductors.

While the invention has been particularly described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A method for aflixing ohmic contacts to a ferromagnetic semiconductor body comprising the steps of:

(a) depositing a small amount of a low melting alloy comprising a rare earth metal and a metal having conducting properties onto the surfaces of said ferromagnetic semiconductor body,

(b) heating said ferromagnetic semiconductor body and said alloy at the melting point temperature of said alloy to cause said alloy to melt, and

(c) continuing said heating to cause said alloy to partially diffuse into said ferromagnetic semiconductor body.

2. A method according to claim 1 wherein said rare earth metal is selected from the group consisting of Ce, Pr, Nd, La, Gd, Tb, Dy, Ho, =Er, Tm, Y and Lu and said metal is selected from the group consisting of Au, Ag, Cu, Zn, Ni, Co, Pd and Pt.

3. A method according to claim 1 wherein said alloy is composed of 76% by weight of La and 24% by weight of Ag.

4. A method according to claim 1 wherein said alloy is composed of 79.5% by weight of La and 20.5% by weight of Au.

5. A method according to claim 1 wherein said alloy is composed of 86% by weight of La and 14% by weight of Cu.

6. A method according to claim 1 wherein said alloy is composed of 88.5% by weight of La and 11.5% by weight of Zn.

7. A method according to claim 1 wherein said alloy is composed of 84.0% by weight of La and 16.0% by weight of Ni.

8. A method according to claim 1 wherein said alloy is composed of 84.0% by weight of La and 16.0% by weight of Co.

9. A method according to claim 1 wherein said alloy is composed of 82.5% by Weight of Pr and 17.5% by weight of Cu.

10. A method according to claim 1 wherein said alloy is composed of 77.9% by weight of Pr and 22.1% by weight of Au.

11. A method according to claim 1 wherein said alloy is composed of 93.0% by weight of Pr and 7.0% by weight of Ni.

12. A method according to claim 1 wherein said alloy is composed of 78.0% by weight of Pr and 22.0% by weight of Ag.

13. A method according to claim 1 wherein said alloy is composed of 74% by weight of Y and 26.0% by weight of Cu.

References Cited UNITED STATES PATENTS 3,376,157 4/1968 Guerci et al. 117-106 3,451,845 6/1969 Schiller 117-201 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 

